排序方式： 共有9条查询结果，搜索用时 234 毫秒
1
1.
The Quaternary Acatlán Volcanic Field (AVF) is located at the western edge of the Trans-Mexican Volcanic Belt (TMVB). This region is related to the subduction of the Pacific Cocos and Rivera plates beneath the North American plate since the late Miocene. AVF rocks are products of Pleistocene volcanic activity and include lava flows, domes, erupted basaltic andesite, trachyandesite, trachydacite, and rhyolite of calc–alkaline affinity. Most rocks show depletion in high field-strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction-related volcanic arcs. 87Sr/86Sr values range from 0.70361 to 0.70412, while Nd values vary from +2.3 to +5.2. Sr–Nd isotopic data plot along the mantle array. On the other hand, lead isotope compositions (206Pb/204Pb=18.62–18.75, 207Pb/204Pb=15.57–15.64, and 208Pb/204Pb=38.37–38.67) give evidence for combined influences of the upper mantle, fluxes derived from subducted sediments, and the upper continental crust involved in magma genesis at AVF. Additionally δ18O whole rock analyses range from +6.35‰ in black pumice to +10.9‰ in white pumice of the Acatlán Ignimbrite. A fairly good correlation is displayed between Sr as well as O isotopes and SiO2 emphasizing the effects of crustal contamination. Compositional and isotopic data suggest that the different AVF series derived from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source. 相似文献
2.
Yardangs in terrestrial ignimbrites: Synergistic remote and field observations on Earth with applications to Mars 总被引：1，自引：0，他引：1
The conditions of formation and the form of yardangs in ignimbrites in the Central Andes of Chile, Bolivia, and Argentina may be the most convincing terrestrial analog to the processes and lithology that produce the extensive yardangs of the Medusae Fossae Formation (MFF) of Mars. Through remote and field study of yardang morphologies in the Central Andes we highlight the role that variable material properties of the host lithology plays in their final form. Here, ignimbrites typically show two main facies: an indurated and jointed facies, and a weakly to poorly indurated, ash- and pumice-rich facies. Both facies are vertically arranged in large (erupted volume >100's of km3) ignimbrites resulting in a resistant capping layer, while smaller (10's of km3) ignimbrites are made predominantly of the weakly indurated facies. The two facies have quite different mechanical properties; the indurated facies behaves as strong rock, fails by block collapse and supports steep/vertical cliffs, while the non-indurated facies is more easily eroded and forms gentle slopes and manifests as more subdued erosional forms. In response to aeolian action, the presence of an upper indurated facies results in large, elongate, high aspect ratio (1:20-1:40) megayardangs that form tall (100 m), thin ridges with steep to vertical walls. These are built on a broad apron of the weakly indurated facies with abundant fallen blocks from the upper indurated facies. These terrestrial megayardangs appear to be analogous to megayardangs with associated block fields seen on Mars. Smaller-volume, weakly indurated ignimbrites are sculpted into smaller, stubbier forms with aspect ratios of 1:5-1:10 and heights rarely exceeding 10 m. Excavation of a windward basal moat suggests an erosional progression like that seen in incipient yardangs on Mars. Excavation rates of 0.007-0.003 cm/year are calculated for the weakly indurated ignimbrites. While a persistent strong unidirectional wind is the dominant parameter controlling yardang formation and orientation, a role for flow separation and vorticity is also suggested by our observations at both yardang types. While the indurated facies is commonly pervasively jointed, jointing is of secondary importance in controlling yardang orientation. Serrated margins, a common feature on Mars, result from oblique intersections of jointing with yardang flanks or scarps of ignimbrite. The processes of yardang formation we describe from ignimbrites from the Central Andes are not necessarily specific to ignimbrites, but do connote that degree and distribution of induration is a major control in yardang formation and this has implications for the lithology of the MFF on Mars. 相似文献
3.
The Citlaltépetl Ignimbrite records one of the largest explosive events during the Holocene activity of Citlaltépetl Volcano
(Pico de Orizaba). Multiple pyroclastic flow units, a fall deposit, and some lahar units were emplaced between 8500–9000 y
B.P. as a result of repetitive but discrete explosive events. The whole ignimbrite resulted from discrete fluctuations in
eruptive intensity that decreased with time. The initial pyroclastic flow pulse was by far the most violent and widespread
event, and its deposits show conspicuous variations in structure and texture that could be associated with different mechanisms
of transport and emplacement. Subpopulation Sequential Fragmentation Transport (SFT) analyses were carried out in order to
determine the physical mechanisms that selectively concentrate or remove particles in the moving flows. We suggest that lateral
and temporal changes in the flow rheology, in which fluidization, yield strength, entrainment of atmospheric air, and sedimentation
played a dominant role in flow propagation and emplacement, may imprint a unique signature in the grain-size spectra. The
lowermost unit of the Citlaltépetl Ignimbrite can be envisaged by a model in which progressive aggradation near the vent became
replaced by en masse emplacement farther outward.
Received: 24 March 1998 / Accepted: 9 October 1998 相似文献
4.
Neoproterozoic magmatism in southern Brazil is associated with translithospheric shear belts and strike-slip basins in a post-collisional setting related to the last stages of the Brasilian-Pan African Orogenic Cycle. It evolved from an association of high-K calc-alkaline, leucocratic-peraluminous and continental tholeiitic magmas, to an association with shoshonitic magmas and, eventually, to an association with magmas of the sodic mildly alkaline series. This magmatism varies from metaluminous to peralkaline and exhibits alkaline sodic affinity. A large volcanism is related to this alkaline sodic magmatism and is named the Acampamento Velho Formation. This unit was coeval with subaerial siliciclastic sedimentation in post-collisional basins preserved in the region. The Acampamento Velho Formation consists of pyroclastic and effusive volcanic deposits, which are mainly silicic, emplaced under subaerial conditions. The best exposures of this volcanism occur on the Ramada and Taquarembó plateaus, located southwest of Rio Grande do Sul in southernmost Brazil. The pyroclastic flow deposits are composed mainly of juvenile fragments such as pumices, shards and crystal fragments. Welding is very effective in these units. High-grade ignimbrites occur at the base and intermediate portions of the deposits and rheoignimbrites are observed at the top. The pre-eruptive temperature calculations, which were obtained at the saturation of zircon, revealed values between 870 °C and 978 °C for Taquarembó Plateau and 850 °C–946 °C for Ramada Plateau. The calculated viscosity values vary from 6.946 to 8.453 log η (Pas) for the rheoignimbrites and 7.818 to 10.588 log η (Pas) for the ignimbrites. Zr contents increase toward the top of the pyroclastic sequence, which indicates an increase in peralkalinity and determines the reduction in viscosity for clasts at the upper portions of the flows. The patterns of the structures of the ignimbrites and rheoignimbrites in the Taquarembó and Ramada plateaus accords well with successive pyroclastic flows that halts en masse. In this model the entire pyroclastic flow halts en masse, so complex vertical changes in grain size and composition are interpreted as recording deposition from successive discrete pyroclastic flows. The stratification observed in intermediate units in Taquarembó Plateau might reflect in this case variation in eruptive dynamics and short pauses. 相似文献
5.
The Reporoa Caldera occupies the northern end of the Reporoa Depression, previously described as a tectonic fault-angle depression. Earlier confirmation of the topographic basin as a caldera had been hindered by the lack of an associated young pyroclastic flow deposit of large enough volume to have caused caldera collapse. New exposures on the eastern margin of the Reporoa basin reveal thick lithic lag breccias (>30 m) interbedded within the 0.24 Ma Kaingaroa Ignimbrites. These ignimbrites were previously attributed to the adjacent Okataina Volcanic Centre. Lag breccia thicknesses and maximum clast sizes decrease rapidly outward from the caldera rim, and discrete breccias are absent from ignimbrite sections more than 3 km from the rim. The lithic lag breccias, together with structural and geophysical evidence, confirm Reporoa Caldera as the source of the c. 100 km3 Kaingaroa Ignimbrites, adding another major rhyolitic volcanic centre to the seven previously recognized in the Taupo Volcanic Zone. Other, older, calderas may also be present in the Reporoa Depression. 相似文献
6.
Throughout most of its geological evolution Etna has been characterized by the eruption of lava flows of a predominantly hawaiitic composition, but within the stratigraphical record there are four major sequences of pyroclastic materials: the Acireale tephra and lahars (˜100000 B.P.); the ‘lower tephra’ and Milo lahars (both ˜26000 B.P.); the Biancavilla ignimbrites (15–15500 B.P.) and the ‘upper tephra’ (˜5000–6000 B.P.). This paper reports investigations carried out on these deposits in order to determine their stratigraphy, petrology, sedimentology, and likely origins. Whereas the Biancavilla ignimbrites were generated when a more evolved, gas-charged magma (benmoreite) was being produced by the volcano, the other suites of pyroclastic deposits were erupted from hawaiitic magmas—similar to those that have characterized the volcano during historical times. These deposits resulted from two processes: violent strombolian activity producing lapilli-rich. coarse, but well-sorted sediments, and hydrovolcanism when the mixing of water and magma in the conduit, brought about more violently explosive activity, giving rise to highly fragmented, poorly sorted, airfall tephra and lahars. Conditions favouring hydrovolcanism occurred at times in the volcano's history when palaeoenvironment and palaeogeography were conducive to the retention of large amounts of surface and subsurface water. Although climates favouring the retention of water at high levels on the volcano have occurred on many occasions in the history of the volcano, at ˜26.000 and ˜5000-6000 B.P. these occurred in conjunction with a construct of sufficient height and suitable configuration to allow storage of water and give rise to hydrovolcanic activity. The nature of the mechanisms responsible for the emplacement of these hydrovolcanic deposits is considered and it is concluded that airfall is the most probable process. Finally, the implications of this research for the assessment of hazard are reviewed. 相似文献
7.
Tasha M. Black Philip A. R. Shane John A. Westgate Paul C. Froggatt 《Bulletin of Volcanology》1996,58(2-3):226-238
Large volume (100–1000 km3), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most
active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites
difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic
successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the
non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma),
Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses
show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we
have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases.
The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of 40Ar/39Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent
magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite
can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped
members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data
indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with
the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted
during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western
United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of
caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed
by the formation of the Okataina and Reporoa calderas in rapid succession.
Received: 20 November 1995 / Accepted: 8 May 1996 相似文献
8.
El Hassan Abia Hassan Nachit Christian Marignac Abderrahmane Ibhi Saïd Ait Saadi 《Journal of African Earth Sciences》2003,36(4):251-271
The Bou Madine ore deposit is located SW of Jbel Ougnat, the easternmost inlier of the Anti-Atlas Pan-African belt in Morocco. The host rocks are high-K calc-alkaline volcanic rocks, that are part of the Neoproterozoic Tamerzaga-Timrachine Formation (TTF, lower PIII). The TTF consists of ignimbrites of rhyolitic to dacitic compositions, andesite flows and hypovolcanic bodies (andesite dykes and rhyolite chonoliths) emplaced along N160°E tension gashes associated with a regional N30°E sinistral fault system. The mineralization is related to a high enthalpy geothermal system, eventually evolving into a low temperature epithermal system. A regional propylitisation (T around 260 °C) overprinted the TTF rocks prior to the emplacement of the mineralization. There were two main hydrothermal stages. During the first stage, massive veins with pyrite, arsenopyrite and minor pyrrhotite and cassiterite were formed. The veins were emplaced along N160°E-trending en echelon joints related to N120°E dextral arrays. A quartz-sericite-pyrite alteration overprinted the propylites around the veins (“bleached haloes”), at temperatures up to 300–310 °C. The second stage of mineralization was coeval with dextral re-activation of the N160°E veins, in relation with a NE-ward shift of the shortening direction. First, polymetallic sulphides (sphalerite, chalcopyrite, stannite, galena) were deposited at temperatures 260 °C. Younger quartz veinlets contain arsenopyrite and minor micrometre-size sulphides and sulpho-salts, hosting the precious metals. This was the low temperature epithermal stage (≈150 °C), in relation with invading meteoric water. 相似文献
9.
The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern
California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera,
caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions
of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites
and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among
the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole
magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped
caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous,
pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one
eruption are characterized by more than one caldera.
Received: 20 December 1997 / Accepted: 15 October 1998 相似文献
1